Thursday, July 31, 2014

Selected Readings on Human Health, Disease, and the Environment

The following
are responses and summaries of various readings related to Human Health,
Disease, and the Environment.

Linkages Between Climate, Ecosystems, and Infectious Disease

Weather and climate are different
ideas in similar fields of study.
Weather is the measurement of temperature, humidity, precipitation, and
wind daily whereas climate measures these factors over a long period of time. Climate is an average and varies with
geography and time. The average
temperature of an area decreases by 6.5 degrees C with every 1000 meter change
vertically and decreases 5 degrees C with every 1000 kilometer change from the
equator. The range with which
temperature fluctuates due to the seasons is greater towards the poles, with
much less change experienced near the equator.
There are many climate cycles occurring from yearly to millennially and
some occurring as little as every 100,000 years; these oscillations can cause
temperature shifts as high as 10 degree C.
Since the Industrial Revolution, and more notably in the last twenty
years, the global average temperature has begun to increase yearly and is
predicted to continue increasing several degrees in the next century. The basis for good predictive weather and
climate models does not rest solely on the atmosphere but also in surface
variations, especially the oceans.
Currently weather prediction is only effective on a timescale of less
than two weeks, while climate models are limited to seasonal or greater
timescales. Including the effects of
ocean, atmosphere, and land may make climate models sufficient for shorter
timescales and weather prediction for greater timescales. With infectious disease, the number of cases
that indicates and epidemic will vary with each particular disease, region, and
time. Emerging disease can be due to
climatic or ecological change, which causes a greater number of people to come
into contact with a natural reservoir of infection. Diseases can be transmitted either directly,
due to contact with an agent, or indirectly, through a natural reservoir or
host. Many studies of infectious disease
use the SEIR framework: S. The proportion of people who are able
to become infected E. Within that
same group, those who have had contact with the agent but who have not become
infected I. The number of people within S group who have
contracted the disease R. The number
of people who have been removed from the original group S due to either death
or immunity. SEIR attempts to account for the many
different factors in disease studies such as population size and density,
demographics, connectivity patterns, and immunity. Diseases and natural reservoirs have
environmental conditions that are both favorable and not for the spread of
particular diseases which include precipitation, temperature, humidity, and
ultraviolet radiation. Most diseases are
sensitive to changes in temperature with regards to their dissemination, replication,
and movement. Climate change over the
long-term may result in increased intensity and occurrence in extreme weather
events, where the direct impact is obviously more deaths due to disasters,
though potentially and indirectly an increase in infectious disease outbreaks. The diseases associated with flooding are
well known while droughts are not nearly as studied, though a disease would not
affect a crisis area should it not have been present in the system before the
extreme event and had not be introduced.
There are many other factors that influence disease dynamics including
land use, migration of disease and hosts, the societal makeup of populations,
and public health services.

Climate Influences on Specific Diseases

Dengue fever is currently the
biggest disease worldwide that is transmitted via insects. It causes fever, headache, muscle and bone
pain, and hemorrhage in humans with about five percent of its cases being
fatal. The tropical regions are the most
affected by dengue with about 50 million occurrences of infection every year. The mosquito is effective as a vector because
it is able to both infect humans and acquire the disease from already infected
humans, thereafter replicating in the stomach of the mosquito. The mosquito finds stagnant water, especially
in manmade containers the most appealing for breeding, thus areas with much
excess garbage and rainfall are most susceptible, though rainfall is not
requisite should the containers be filled by people. Temperature is the strongest factor in dengue
fever, because the mosquito biting frequencies are much lower in cooler times,
breeding and hatching is much less frequent, if at all possible, and the virus
itself takes longer to form in the gut of the mosquito. An increase in temperature by as little as
one degree could see the at risk population increase by as much as fifty
percent in areas already containing the dengue fever.

Malaria is a disease transmitted
via mosquito in the tropics which causes chills and fever and potentially
death. Upward of 300 million cases are reported
yearly with around one million deaths. Ninety
percent of cases are found in sub-Saharan Africa, with most of the remainder
taking place in the tropics. Malaria is
on the rise due to poor control of mosquito populations, insecticide
resistance, and drug resistant strains appearing in recent years. Malaria acts very similarly to dengue fever
in that higher temperature generally results in more cases due to the behavior
of the carrying mosquitoes and its virus.
The spread of malaria seems most affiliated with economics, considering
that most of the cases occur in areas that have poor sanitation and life
style.

St. Louis encephalitis is an
inflammation of the brain due to a virus carried by mosquitoes. St. Louis encephalitis can cause headache, fever,
stiffness, stupor, disorientation, coma, and paralysis. In the United States there are about 128
cases on average every year. The disease
is dominant in the southern U.S. where the temperature is generally warmer and
more prevalent in years of greater rainfall or in areas of recent
flooding.
Rift Valley Fever can cause abortions and death in the young of animals, while
causing fever, headache, myalgia, and incapacitation in people, with one
percent of cases being fatal. Rift
Valley Fever is transmitted by mosquitoes, mostly on the African continent,
after periods of great rainfall where they generally only infect animals.

Hantavirus causes fever, muscle
aches, and respiratory problems, which all develop over a period of less than a
week after initial infection. The
largest outbreak of the virus in the last fifteen years saw 80 cases in
1993. The virus is transported in the
urine, feces, and saliva of deer mice in the southwestern United States. The virus is greatly influenced by rainfall
with large population jumps in years directly following big rain seasons.

Lyme disease is a bacteria
transmitted by ticks which can cause fever, fatigue, headache, and muscle and
joint aches, though it is rarely the primary cause of death. It is most prevalent in the suburbs, or in
areas near new forest. The ticks
generally feed on field mice and deer, which have found abundance in the areas
of new forests which only fifty years ago were home to farms and industry thereby
leaving a vacant lot for new life to emerge.
There is currently insufficient data to conclude what effect the climate
has on Lyme disease and its vectors.

Influenza is a virus of the
respiratory tract in humans. The virus
is transmitted through the air, enabling it to move quickly through
populations. Millions of cases are
identified every year, with three major outbreaks occurring in just the last
century. Currently there is no human
immunity to influenza due to the main strain regularly incorporating pieces of
the infected DNA into its genome.
Infection seems to be more prevalent during colder months and in areas
of the world closer to the poles than the equator, though it has not been shown
to be directly influenced by temperature; many theories include that people
crowding inside in the wintertime enables for easier transmission and also that
there is a greater humidity in the winter months which influences the
virus.

Cholera is a bacteria causing
diarrheal disease in people. Cholera is
most often transmitted through the water supply, though modern filtration and
chlorination techniques have virtually eliminated the disease in developed
countries. The developing and
third-world nations contribute to almost all the cases of cholera today. Increased temperature, or temporary climate
shifts such as El Nino, have shown an increase in the number of cholera
cases.

The Hantavirus is transmitted
through the air, via the excretions of rodent urine, feces, and saliva and
through rodent bites. The virus was
previously referred to as the Korean hemorrhagic fever, though that term is in
use no longer. The disease was initially
recognized by modern Western medicine during the Korean war in the 1950s and
recognized in its current species form in 1993 during an outbreak of the “Sin
Nombre virus” in the Four Corners region of the United States. The virus is found most often in from China
up to Russia, Northern Europe, South America, and North America. In the United States the virus is dominant in
the Southwestern states, including Texas and California, as well as the
Northwestern states Montana, Idaho, and Utah.
The Hantavirus has an incubation time of two to four weeks in
humans. Symptoms include fever, chills,
headaches, nausea, stomach and back pain, respiratory problems, and
gastro-intestinal problems. The disease
can cause renal failure, hypoxemia, and tachycardia. The virus is most occurrent in years after
excess rainfall when the mouse and rat populations thrive due to the excess
food put off by the area’s plant life.
Housing areas near forests and in rural areas are most susceptible to
the disease due to the increased likelihood of rodent infestation and life
within a home. Increased flooding and
equally increased sunlight during moist times has shown an increase in
Hantavirus occurrences because of increased food for rodent populations in the
areas.

With climate change extreme weather
events are expected to increase in both intensity and number and have the
greatest impact on poorer communities and nations. The ENSO cycle, which occurs twice per ten
years, influences much of the world’s weather and climate change will likely
increase how often this occurs and its amplitude. Warmer temperatures will increase the breeding
of mosquitoes and allow for more rapid maturation of pathogens within the
organism. An increase in temperature
could cause for mosquito carried diseases to have a greater geographic range of
infection. While an increase in extreme
events such as flooding could also increase the occurrence of mosquito vector
disease as well as rodent spread diseases and an increase in diarrheal
diseases. Climate change will come with
more often and stronger heat waves, warmer summers, and less cold winters. Heat wave related deaths is predicted to
increase many times its current rate over the next fifty years, though the
decrease in the number of cold related deaths may outweigh the heat related
deaths. The number of natural disasters
and extreme weather events is on the rise, with nearly triple the number of
events occurring annually when compared with the number in the 1960s while the
number of people affected by these events is proportionally increasing. It is estimated that the risk of diarrheal
disease will increase by up to ten percent over the next twenty years due to
climate change. Proper modeling
techniques need to be further perfected in order to predict more clearly.

Readings above may have been drawn from the
following sources:

Six
Modern Plagues and How We are Causing Them, Mark Jerome Walters; Shearwater Books, 2003, ISBN 155963992X